Post on 13-Aug-2015
A
Summer Intern Project Report
On
HEAT TREATMENT
at
Submitted by
NEERAJ VIJAYVARGIYA ID No: 2010UMT258
of
BACHELOR OF TECHNOLOGY
in
METALLURGICAL & MATERIALS ENGINEERING
Malaviya National Institute of Technology
Jaipur
July, 2013
2
ACKNOWLEDGEMENTS
First and Foremost, I would like to thank Vardhman Special Steels Limited for giving me an
opportunity to accomplish my Project. Working here has been a great learning experience.
I take this opportunity to express a deep sense of gratitude to Mr. Mal Singh Rathore, for
assigning to me an interesting project and for his helpful guidance, criticism, encouragement
throughout the every stage of this study.
I would also like to express my profound gratitude to Mr. B. D. Chawla (VP Tech.), Mr. Rishu,
Mr. Vikram Mahajan, Mr. Pawan Kishore, Mr. Tilak Raj and Miss. Amrit Jallawalia for their
cordial support, valuable information and guidance throughout the course of this project.
I am obliged to all members of the Metallurgical Services Department of VSSL for the valuable
information provided by them in their respective fields. I am grateful for their cooperation during
the period of my project.
Finally, I wish to express my gratitude to my family for their complimentary love and unshakable
faith in me during my life.
3
ABSTRACT
This document describes the various heat treatment processes which are being done or to be done
in future at Vardhman Special Steels Limited. Study of various spherodising cycles which are
being used at VSSL is analyzed and study is done to reduce the cycle time for the same.
Effect of the alloying elements on hardness(before and after spherodisation) is studied from the
data of 5 months and conclusion was made to reduce the percentage of alloying element keeping
the aim and standard chemistry in mind to reduce the cycle time.
Full annealing and normalizing are also discussed in this report along with the problem of ferrite
banding. Reduction in banding to a great extent due to application of magnetic field is also studied
and conclusions were made.
Apart from this, futuristic projects of VSSL like quenching and tempering are also discussed in
short.
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TABLE OF CONTENTS
ACKNOWLEDGEMENT……………………………………....………….....…...2
ABSTRACT……………………………………………………………………......3
TABLE OF CONTENTS…………………………………………………………..4
1. INTRODUCTION
1.1 Introduction to heat treatment…………………………………………………..5
1.2 Vardhman Special Steels Limited………………………………………………6
2. PURPOSE OF HEAT TREATMENT……………………………………………7
3. TYPES OF HEAT TREATMENT ……………………………………………….9
3.1 Normalising………………………………………………………………………9
3.2 Full Annealing…………………………………………………………………..13
3.3Problem of ferrite banding in full annealing…………………………………….15
a.) Cause…………………………………………………………………………….15
b.) Remedies………………………………………………………………………...16
C.) Conclusion………………………………………………………………………17
3.4 Process annealing………………………………………………………………..17
3.5 Stress relief annealing…………………………………………………………...18
3.6 Spheroidizing……………………………………………………………………..18
a) Various methods of spheroidizing………………………………………………….19
b) Some of the cycles which are being used at VSSL……………………………….22
c) Conclusions based upon the study of spherodisation from Jan’13 to May’13 at VSSL..24
3.7Quenchig…………………………………………………………………………..25
a) Types of quenching………………………………………………………………...26
3.8 Tempering and various stages of tempering……………………………………...27.
4. REFERENCES…………………………………………………………………….29
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INTRODUCTION
1.1Introduction to heat treatment
A STEEL is usually defined as an alloy of iron and carbon with the carbon content between a few
hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5
wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels and stainless
steels. Steels can exhibit a wide variety of properties depending on composition as well as the
phases and microconstituents present, which in turn depend on the heat treatment.
Heat treatment is an operation or combination of operations involving heating at a specific rate,
soaking at a temperature for a period of time and cooling at some specified rate. The aim is to
obtain a desired microstructure to achieve certain predetermined properties.
Steels can be heat treated to produce a great variety of microstructures and properties. Generally,
heat treatment uses phase transformation during heating and cooling to change a microstructure in
a solid state.
Heat Treatment is often associated with increasing the strength of material, but it can also be used
to alter certain manufacturability objectives such as improve machining, improve formability, and
restore ductility after a cold working operation. Thus it is a very enabling manufacturing process
that can not only help other manufacturing process, but can also improve product performance by
increasing strength or other desirable characteristics.
Figure: Various heat treatment processes
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1.5 Vardhman Special Steels limited
Vardhman Special Steels limited is a part of the Vardhman Group of Companies, over 24
manufacturing facilities in five states across India. The Group business portfolio includes Yarn,
Grieve & processed Fabric, Garments, Sewing Thread, Acrylic Fiber and Alloy Steel. It is the
largest producer and exporter of Yarns and Grey woven Fabrics from India. Vardhman is also the
largest producer of Tyercord yarns and the second largest producer of sewing Threads in India.
Vardhman Special Steels limited is one of the secondary steel making companies that produces
Special and Alloy steel long products that find end use in the Automotive, Tractor and other
Engineering sectors. The company is an approved source of steel for several Original Equipment
Manufacturers. Vardhman Special Steels comprises a Steel Melt shop having a 30 ton UHP
Electric arc furnace with EBT, Ladle Refining Furnace, and Vacuum Degassing unit. Company
has a three strand Continuous Casting Machine, a rolling mill and a Bright Bar Unit facility.
It was Vardhman Group's faith in the economy of the country, specifically in core industrial sector
that initiated the Group's venture into the steel industry. The story of Vardhman Special Steels
began way back in the year 1972 with the commissioning of Oswal Steels at Faridabad to
manufacture Special & Alloy Steels; with an initial capacity of 50,000 Metric tonnes per
annum.1986 was the turning point in the history of Vardhman Special Steels. The Company
acquired another plant at Ludhiana, which was later upgraded with the latest state-of-the-art
technology to an installed capacity of 1, 00,000 Metric tonnes per annum.
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2. PURPOSE OF HEAT TREATMENT
Various types of heat treatment processes are used to change the following properties or
conditions of the steel:
Improve the toughness
Increase the hardness
Increase the ductility
Improve the machinability
Refine the grain structure
Remove the residual stresses
Improve the wear resistance
The following are the general reasons for heat treatment:
Hardening (Steels can be heat treated to high hardness and strength levels. The reasons for doing
this are obvious. Structural components subjected to high operating stress need the high strength
of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices
need a hardened structure to resist wear and deformation.)
Tempering (As-quenched hardened steels are so brittle that even slight impacts may cause
fracture. Tempering is a heat treatment that reduces the brittleness of a steel without significantly
lowering its hardness and strength. All hardened steels must be tempered before use.)
Softening a Hardened Structure (Hardening is reversible. If a hardened tool needs to be
remachined, it may be softened by heat treatment to return it to its machinable condition. Most
steels weld better in their soft state than in their hardened state; softening may be used to aid
weldability.)
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Recrystallization (If a metal is cold worked, grains or crystals deform, become elongated, and in
doing so harden and strengthen a metal. There is a limiting amount of cold work that a particular
metal can be subjected to. In rolling of steel into thin sheets, you can only reduce the cross-
sectional area so much before it gets too hard to roll. At this point it would be desirable to return
the grains to their original shape. Heat treatment can accomplish this. The transformation of cold-
worked grains to an undistorted shape is called recrystallization. Very large coarse grains can also
be refined by recrystallization.This type of heat treatment is essential if a steel is to be subjected to
severe cold working in rolling, drawing, etc.)
Stress Relief (One of the most frequent reasons for heat treatment is to remove internal stress
from a metal that has been subjected to cold working or welding. Stress relieving is a heat
treatment used to remove internal strains without significantly lowering the strength. It is used
where close dimensional control is needed on weldments, forgings, castings, etc.)
Hot-Working Operations (Most metal shapes produced by steel mills are at least rough shaped at
elevated temperatures. Heat treating is required to bring the rough metal shapes to the proper
temperature for hot-forming operations.Forging, hot rolling, roll welding, and the like are all
performed at temperatures of sufficient magnitude as to prevent the formation of distorted grains
that will harden the metals. Hot-working operations require dynamic recrystallization which is
achieved by working at the proper hot-work temperatures.)
Diffusion of Alloying Elements (One of the criteria for hardening a steel is that it have sufficient
carbon content. Low carbon steels can be hardened, at least on the surface, by heat treating at an
elevated temperature in an atmosphere containing an alloying element that will diffuse into the
steel and allow surface hardening on quenching. Carbon is frequently diffused into the surface of
soft steels for surface hardening. Using this same principle, elements such as chromium, boron,
nitrogen, and silicon can be diffused in the surface of a steel for special purposes.)
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3.TYPES OF HEAT TREATMENT
Common heat treatment processes:
Normalizing
Spheroidising
Full annealing
Process annealing
Stress relieving
Direct Hardening
Tempering
Austempering
Martempering
Diffusion Hardening
Selective hardening
3.1 NORMALIZING
NORMALIZING OF STEEL is a heat-treating process that is often considered from both thermal
and microstructural standpoints. In the thermal sense, normalizing is an austenitizing heating cycle
followed by cooling in still or slightly agitated air. Typically, the work is heated to a temperature
about 55 °C (100 °F) above the upper critical line of the iron-iron carbide phase diagram, that is,
above Ac3 for hypoeutectoid steels and above Acm for hypereutectoid steels. To be properly
classed as a normalizing treatment, the heating portion of the process must produce a
homogeneous austenitic phase (face-centered cubic, or fcc, crystal structure) prior to cooling.
Figure : Normalizing temperatures for hypoeutectoid and hypereutectoid steels.
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The following is the list of the reasons for normalizing the steel :
To produce a harder and stronger steel than full annealing
To improve the machinability
To modify and refine the grain structure
To obtain a relatively good ductility without reducing the hardness and strength
Figures a, b, c and d show the effect of annealing and normalizing on the ductility, tensile
strength, hardness and yield point of steels.
Figure a: Ductility of annealed and normalized steels.
As indicated in Figure (a), annealing and normalizing do not present a significant difference on the
ductility of low carbon steels. As the carbon content increases, annealing maintains the %
elongation around 20%. On the other hand, the ductility of the normalized high carbon steels drop
to 1 to 2 % level.
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Figure b : Tensile strength of normalized and annealed steels.
Figure c: yield point of annealed and normalized steels
Figures (b) and (c) show that the tensile strength and the yield point of the normalized steels are
higher than the annealed steels. Normalizing and annealing do not show a significant difference on
the tensile strength and yield point of the low carbon steels. However, normalized high carbon
steels present much higher tensile strength and yield point than those that are annealed.
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Figure d: Hardness of normalized and annealed steels.
As seen from Figure (d), low and medium carbon steels can maintain similar hardness levels when
normalized or annealed. However, when high carbon steels are normalized they maintain higher
levels of hardness than those that are annealed.
3.2 Full Annealing
A common annealing practice is to heat hypoeutectoid steels above the upper critical temperature
(A3) to attain full austenitization. The process is called full annealing. In hypoeutectoid steels
(under 0.77% C), supercritical annealing (that is, above the A3 temperature) takes place in the
austenite region (the steel is fully austenitic at the annealing temperature).
However, in hypereutectoid steels (above 0.77% C), the annealing takes place above the A1
temperature, which is the dual-phase austenite-cementite region. In general, an annealing
temperature 50 °C (90 °F) above the A3 for hypoeutectic steels and A1 for hypereutectoid steels is
adequate.
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Figure: Full annealing range
Full annealing process consists of three steps. First step is heating the steel component to above A3
(upper critical temperature for ferrite) temperature for hypoeutectoid steels and above A1 (lower critical
temperature) temperature for hypereutectoid steels by 30-500
C .
The second step is holding the steel component at this temperature for a definite holding (soaking)
period of at least 20 minutes per cm of the thick section to assure equalization of temperature
throughout the cross-section of the component and complete austenization. Final step is to cool the
hot steel component to room temperature slowly in the furnace, which is also called as furnace
cooling. The full annealing is used to relieve the internal stresses induced due to cold working,
welding, etc, to reduce hardness and increase ductility, to refine the grain structure, to make the
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material homogenous in respect of chemical composition, to increase uniformity of phase
distribution, and to increase machinability.
3.3 Problem of ferrite banding in full annealing:
Ferrite banding: Parallel bands of free ferrite aligned in the direction or working. Sometimes
referred to as ferrite streaks.
Figure: Pearlite and ferrite band in a medium carbon steel
This is a medium carbon steel, consisting of ferrite and pearlite (a lamellar mixture of ferrite and
cementite). It is banded (dark bands = pearlite; light bands = ferrite) an undesirable condition.
As the piece cooled below the all-austenite region within which the hot rolling had been
performed, ferrite was first precipitated at the austenite grain boundaries, thereby causing carbon
to be rejected into the remaining austenite. Once the piece cooled below the eutectoid temperature
(727C) the 0.8% carbon austenite transformed discontinuously via coupled growth of ferrite and
cementite in a lamellar (layered) morphology called pearlite because of its appearance to the
unaided eye after etching.
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a) Cause:
The banding is the result of microsegregagtion of manganese during solidification of the original
sample. Slow cooling in annealing makes ferrite formation preferentially in the manganese
depleted regions, i.e., produces bands of ferrite where Mg is low, and bands of pearlite where Mg
is high.
As this piece cooled from the all-austenite region, alpha ferrite was precipitated - more so, the less
the local manganese content. Therefore, the last areas to transform were richer in both manganese
and carbon, so those regions are now mostly pearlite.
b) Remedies:
1.) Banding can be eliminated by prolonged heating and/or extensive hot working to homogenize
the metal with respect to the manganese, or it can be circumvented by short time, high temperature
austenitization, which levels out the local carbon content but not the manganese variations.
2.)Magnetic annealing can be done in which magnetic field is applied parallel to hot rolling
direction of rods.A magnetic field of 14T was applied in the experiment** .
A cooling rate of
46oC/Min can be achieved for full annealing.
Figure : Microstructure of 42CrMo steel after being austenitized at 860oC for 30 min and cooled naturally at about
1oC/min (the hot-rolling direction is horizontal in the micrograph). Bright areas -ferrite grains; dark areas - pearlite
colonies.
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Figure : Microstructure of 42CrMo steel after being austenitized at 880 oC for 33 min and cooled at 46
o_C/min, with
magnetic field of 14 T (the magnetic-field direction is horizontal in the micrograph). Bright areas=ferrite grains; dark
areas=pearlite colonies.
c) Conclusions:
1.) Banding resulted from hot processing can be eliminated upto a very good extent by this
method as seen in the figure.
2.)A very high cooling rate of 46oC/Min can be obtained with desirable microstructure while we
are having cooling rate of 1oC/Min at VSSL.
3.4 Process Annealing
Process annealing is used to treat work-hardened parts made out of low-Carbon steels (< 0.25%
Carbon). This allows the parts to be soft enough to undergo further cold working without
fracturing. Process annealing is done by raising the temperature to just below the Ferrite-Austenite
region, line A1on the diagram. This temperature is about 727 ºC (1341 ºF) so heating it to about
700 ºC (1292 ºF) should suffice. This is held long enough to allow recrystallization of the ferrite
phase, and then cooled in still air. Since the material stays in the same phase through out the
process, the only change that occurs is the size, shape and distribution of the grain structure. This
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process is cheaper than either full annealing or normalizing since the material is not heated to a
very high temperature or cooled in a furnace. Process annealing relieves the internal stresses in the
cold worked steels and weldments, and improves the ductility and softness of the steel. Refinement
in grain size is also possible by the control of degree of cold work prior to annealing or by control of
annealing temperature and time.
3.5 Stress relief annealing
Stress relief annealing process consists of three steps. The first step is heating the cold worked
steel to a temperature between 5000
C and 5500
C i.e. below its recrystallization temperature. The
second step involves holding the steel component at this temperature for 1-2 hours. The final step
is to cool the steel component to room temperature in air.The stress relief annealing partly relieves
the internal stress in cold worked steels without loss of strength and hardness i.e. without change
in the microstructure. It reduces the risk of distortion while machining, and increases corrosion
resistance. Since only low carbon steels can be cold worked, the process is applicable to
hypoeutectoid steels containing less than 0.4% carbon. This annealing process is also used on
components to relieve internal stresses developed from rapid cooling and phase changes.
3.6 Spheroidizing:
Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network
around the pearlite. This presents difficulty in machining the hypereutectoid steels. To improve
the machinability of the annealed hypereutectoid steel spheroidize annealing is applied. This
process will produce a spheroidal or globular form of a carbide in a ferritic matrix which makes
the machining easy. Prolonged time at the elevated temperature will completely break up the
pearlitic structure and cementite network. The structure is called spheroidite. This structure is
desirable when minimum hardness, maximum ductility and maximum machinability are required.
It is also called as Soft Annealing.
The majority of all spheroidizing activity is performed for improving the cold formability of
steels. It is also performed to improve the machinability of hypereutectoid steels, as well as tool
steels. A spheroidized microstructure is desirable for cold forming because it lowers the flow
stress of the material. The flow stress is determined by the proportion and distribution of ferrite
and carbides. The strength of the ferrite depends on its grain size and the rate of cooling. Whether
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the carbides are present as lamellae in pearlite or spheroids radically affects the formability of
steel.
Steels may be spheroidized, that is, heated and cooled to produce a structure of globular carbides in a
ferritic matrix. Figure shows 1040 steel in the fully spheroidized condition.
Figure : Spheroidized microstructure of 1040 steel after 21 h at 700 °C (1290 °F). 4% picral etch. 1000×
a) Spheroidization can take place by the following method
Method 1.)
Prolonged holding at a temperature just below Ae1 and slow cooling.
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Method 2.)
Heating and cooling alternately between temperatures that are just above Ac1 and just below Ar1
Method 3.)
A more common commercial method consists of heating to a temperature of 50°F (13-26°C)
below Ac1, hold at this temperature, then increase the temperature set point between Ac1 and Ac3
and hold again. Following the second soak period, the temperature is decreased slowly.
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Method 4.)
Heating to a temperature just above Ac1, and then either cooling very slowly (30-50 °C/Hr) in the
furnace or holding at a temperature just below Ar1 or just above Ar1.
Method we use at VSSL:
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b) Some of the cycles which are being used at VSSL:
1.)Cycle for 16MnCr5,20MnCr5, 20MnCr5H,16MnCr5H,SAE1020
2.) Cycle for S25C
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3.)Cycle for SAE1010 and AISI1010
c) Conclusions based upon the study of spherodisation from
Jan’13 to May’13 at VSSL:
As we know that
Ac1 ( 0C ) = 723 – 10.7Mn – 16.9Ni + 29.1Si + 16.9Cr + 6.38W + 290As
And Ac3 ( 0C ) = 910 - 203√C – 15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W – 30Mn + 11Cr –
20Cu - 700P - 120As – 400Al- 400Ti
1.) Percentage of alloying elements like Si,Cr,Mn,W,Mo,V which impart hardness to structure can
be reduce somewhat according to aim and standard chemistry for the ease of further
spherodisation.
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3.7 Quenching:
QUENCHING refers to the process of rapidly cooling metal parts from the austenitizing or
solution treating temperature, typically from within the range of 815 to 870 °C (1500 to 1600 °F)
for steel. Stainless and high-alloy steels may be quenched to minimize the presence of grain
boundary carbides or to improve the ferrite distribution but most steels including carbon, low-
alloy, and tool steels, are quenched to produce controlled amounts of martensite in the
microstructure. Successful hardening usually means achieving the required microstructure,
hardness, strength, or toughness while minimizing residual stress, distortion, and the possibility of
cracking.The selection of a quenchant medium depends on the hardenability of the particular
alloy, the section thickness and shape involved, and the cooling rates needed to achieve the
desired microstructure. The most common quenchant media are either liquids or gases. The liquid
quenchants commonly used include:
· Oil that may contain a variety of additives
· Water
· Aqueous polymer solutions
· Water that may contain salt or caustic additives
The most common gaseous quenchants are inert gases including helium, argon, and nitrogen.
These quenchants are sometimes used after austenitizing in a vacuum. The ability of a quenchant
to harden steel depends on the cooling characteristics of the quenching medium.
Quenching effectiveness is dependent on the steel composition, type of quenchant, or the
quenchant use conditions. The design of the quenching system and the thoroughness with which
the system is maintained also contribute to the success of the process.
Fundamentals of Quenching and Quenchant Evaluation
Fundamentally, the objective of the quenching process is to cool steel from the austenitizing temperature
sufficiently quickly to form the desired microstructural phases, sometimes bainite but more often
martensite. The basic quenchant function is to control the rate of heat transfer from the surface of the
part being quenched.
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Types of quenching
· Direct quenching
· Time quenching
· Selective quenching
· Spray quenching
· Fog quenching
· Interrupted quenching
Direct quenching refers to quenching directly from the austenitizing temperature and is by far
the most widely used practice. The term direct quenching is used to differentiate this type of
cycle from more indirect practices which might involve carburizing, slow cooling, reheating,
followed by quenching.
Time quenching is used when the cooling rate of the part being quenched needs to be abruptly
changed during the cooling cycle. The change in cooling rate may consist of either an increase or
a decrease in the cooling rate depending on which is needed to attain desired results. The usual
practice is to lower the temperature of the part by quenching in a medium with high heat
removal characteristics (for example, water) until the part has cooled below the nose of the time
temperature- transformation (TTT) curve, and then to transfer the part to a second medium(for
example, oil), so that it cools more slowly through the martensite formation range. In some
applications, the second medium may be air or an inert gas. Time quenching is most often used
to minimize distortion, cracking, and dimensional changes.
Selective quenching is used when it is desirable for certain areas of a part to be relatively
unaffected by the quenching medium. This can be accomplished by insulating an area to be more
slowly cooled so the quenchant contacts only those areas of the part that are to be rapidly
cooled.
Spray quenching involves directing high-pressure streams of quenching liquid onto areas of the
workpiece where higher cooling rates are desired. The cooling rate is faster because the
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quenchant droplets formed by the high-intensity spray impact the part surface and remove heat
very effectively. However, low-pressure spraying, in effect a flood-type flow, is preferred with
certain polymer quenchants.
Fog quenching utilizes a fine fog or mist of liquid droplets in a gas carrier as the cooling agent.
Although similar to spray quenching, fog quenching produces lower cooling rates because of the
relatively low liquid content of the stream.
Interrupted quenching refers to the rapid cooling of the metal from the austenitizing
temperature to a point above the Ms where it is held for a specified period of time, followed by
cooling in air. There are three types of interrupted quenching: austempering, marquenching
(martempering), and isothermal quenching.
3.8 Tempering and its various stages
In this process quenched component heats or tempers in order to relieve the internal stresses and
reduce the brittleness. During tempering, which is always carried out below the lower critical
temperature, martensite tends to transform to the equilibrium structure of ferrite and cementite.
The higher the tempering temperature the more closely will the original martensitic structure
revert to this ferrite and cementite mixture and so strength and hardness fall progressively, whilst
toughness and ductility increase.Thus by choosing the appropriate tempering temperature a wide
range of mechanical properties can be achieved in carbon steels. The structural changes which
occur during the tempering of martensite containing more than 0.3% carbon, take place in three
stages:
First Stage (100 to 250 °C, or 210 to 480 °F)
First stage occurs at temperatures below 200 °C. This stage involves conversion of the martensite
to low carbon martensite (0.25%C) plus epsilon carbide (є). Є-carbide is metastable and richer in
carbon than cementite and is described by the formula Fe2.5C (or Fe5C2). low carbon martensite
retains some degree of tetragonality because it still contains more carbon in solid solution than
would ferrite; there are no changes in the morphology of the martensite crystals. At this stage a
slight increase in hardness may occur because of the presence of the finely-dispersed but hard є-
26
carbide. Brittleness is significantly reduced as quenching stresses disappear in consequence of the
transformation. At 100°C the transformation proceeds very slowly but increases in speed up to
200°C.
Second Stage (200 to 300 °C, or 390 to 570 °F)
The transformation of retained austenite to ferrite and cementite.
Third Stage
Third stage start at 300°C. At this stage є-carbide begins to transform to ordinary cementite and
this continues as the temperature rises. In the mean time the remainder of the carbon begins to
precipitate from the low carbon martensite also as cementite and in consequence the martensite
structure gradually reverts to one of ordinary BCC ferrite. Above 500°C the cementite particles
coalesce into larger rounded globules in the ferrite matrix. This structure was formerly called
sorbite or tempered martensite. Due to the increased carbide precipitation which occurs as the
temperature rises the structure becomes weaker but more ductile, though above 550°C strength
falls fairly rapidly with little rise in ductility.
Figure. Variation in properties with tempering temperature
27
REFERENCES:
1. Nandita Gupta,S.K. Sen(2006)Defence Science Journal, Vol. 56, No. 4, October 2006, pp. 665-676
2. ASM Metals Handbook Volume 4 : Heat Treating
3. Yudong Zhang, Changshu He, Xiang Zhao, Claude Esling and Liang Zuo(2003)
(A New Approach for Rapid Annealing of Medium Carbon Steels)
4. Sidney H. Avner (Introduction to physical metallurgy,2nd edition)
5. http://info.lu.farmingdale.edu/depts/met/met205/ANNEALING.html
6. Vardhman Special Steels Limited, || http://www.vardhmansteel.com/